This invention relates generally to launch vehicles for placing spacecraft into orbit around the earth and, more particularly, to launch vehicles equipped with lift producing surfaces of sufficient capacity to permit the launch vehicles to be towed as gliders behind conventional aircraft. A launch vehicle so configured may be regarded as "air-launched" by conventional aircraft, or, alternatively as a launch vehicle augmented by a conventional aircraft which serves as a "zero-stage."
A limited number of differing types of launch vehicles is currently available for placing spacecraft into orbit around the earth. Virtually all are launched under rocket power from a fixed launch pad. This limits the rapidity with which launches can be performed to the time required to prepare the launch pad, assemble the launch vehicle on the pad, place the spacecraft on the vehicle, load propellant into the vehicle, verify that its systems are operating properly, and perform the launch. When the requirement arises to place a spacecraft into a specific orbital plane with respect to the fixed stars, the opportunity to launch is limited to a very short time as the orbital plane passes over the launch site. This time, referred to as the launch window, can be as short as a few seconds if the desired orbital plane is highly inclined to the equator and the launch pad is at a low latitude. If any operation leading up to launch is delayed, the launch window may be missed, and the launch may have to be delayed until the next opportunity. The complexity of launch operations is often such that the next passage of the desired orbital plane occurs before the vehicle can be made ready for another attempt. Maintaining a launch crew on site and repeatedly performing pre-launch operations is a significant contributor to the high cost of space launch operations.
Pad-launched vehicles can deliver spacecraft only to certain orbital inclinations by virtue of the geographic location of the launch pad. Safety concerns related to flying over inhabited land masses restrict the direction in which a vehicle can be launched from a given pad, and consequently limit the maximum inclination of the orbit which can be achieved. The minimum inclination which can be achieved from a fixed launch pad is determined by and equal to the geographic latitude at which the pad is situated. Though propulsive maneuvers can be performed to change orbital inclination once the spacecraft is in orbit, the weight of propellant required to do so is prohibitive for changes greater than 5 or so degrees.
Launch pad construction is very costly, as is launch pad maintenance and post-launch refurbishment. These costs are reflected in the cost of launch. The nature of the earth's geography is such that only a small number of remote locations, at the equator, are suitable for launching into orbits of arbitrary inclination. For launch service providers who do not have access to these locations, multiple launch sites at various locations must be built in order to be able to place spacecraft into orbits of arbitrary inclination. The cost of multiple launch sites can be prohibitive, so that launch service providers are unable to afford enough sites to launch into orbits of arbitrary inclination. This results in a restriction of the types of missions that can be performed by a given launch service provider.
A recently implemented improvement in space launch has emerged wherein the launch vehicle is carried on board a conventional aircraft. The aircraft can fly to an arbitrary geographic location, where the launch vehicle is released and propels its payload (spacecraft) into orbit. This operation is referred to as "air-launch," and vehicles so configured as "air-launched."
An alternative way of regarding air-launch, appropriate when applied to launch vehicles capable of taking off from the ground, is to consider the launch aircraft as a "zero-stage." This parlance is commonly used to describe propulsion systems added to existing launch vehicles to augment their performance by raising them to a certain altitude and velocity before the launch vehicle's own propulsion system can be ignited. This reduces the total energy the existing launch vehicle must add to the payload, and translates into either greater payload capacity or into placing the same payload into a more energetic orbit. Reference to the launch aircraft as a "zero-stage" would apply in cases where the launch vehicle is either capable of taking off from the ground under its own power, or where the launch vehicle was not specifically designed to be air-launched.
The advantages of air-launch over ground-launch are numerous. The launch location can be selected so that no inhabited land mass is jeopardized by the vehicle as it flies over, yet the spacecraft can be placed into an orbit of any desired inclination. The variety of missions which can be performed using this aircraft as a launch platform is thus significantly greater than that which can be performed by a vehicle launched from a fixed pad. Moreover, only one aircraft need be purchased, and it can be flown from any conventional airport facility which will permit such operation. This is equivalent to having one "launch pad" (the aircraft) which can be easily moved to any desired geographic location. In the alternative representation of such a system as a launch vehicle having an aircraft as a zero-stage, the equivalence becomes one of having multiple launch pads already in place around the world in the form of the above mentioned conventional airport facilities.
Also, when launching into specific, highly inclined orbits, the aircraft launched vehicle can have a launch window whose duration is limited only by the time the aircraft can remain aloft. This can be accomplished by flying westward at a latitude and speed which permit the aircraft to keep pace with the orbital plane as the earth rotates beneath it. The chances of missing a launch window are thereby significantly reduced.
As mentioned previously, the launch vehicle has to add less potential energy to the spacecraft, since it begins its powered flight at a higher altitude than does a vehicle launched from a ground-based pad. The velocity of the aircraft is also added to that of the launch vehicle, so that the launch vehicle does not have to provide all of the velocity needed to reach orbit. If the launch vehicle is rocket propelled, the performance of the rocket engine can be higher than if it is launched from the ground due to the lower back-pressure on the nozzle at the launch altitude.
Finally, for a given orbital inclination, the launch vehicle may be launched in a due-east direction from a latitude equal to the desired orbital inclination. This adds the velocity of the earth's rotation to the vehicle's initial velocity to the maximum extent possible. These factors all contribute to a vehicle which, for a given launch weight, can place a heavier spacecraft into orbit than it could if launched from the ground, or the same payload into more energetic trajectories.
Even more performance enhancement is gained by adding lifting surfaces to the vehicle. These use aerodynamic forces to augment the thrust produced by the launch vehicle's propulsion system, effectively offsetting the performance loss usually incurred by the propulsion system having to first offset the vehicle's weight before actually providing acceleration.
The sole current aircraft-launched system (Orbital Sciences Corporation's Pegasus.TM.) has wing surface area only sufficient to partially offset the vehicle's weight at the speed of the launch aircraft. As the vehicle accelerates and, at the same time, becomes lighter by virtue of expending propellant, the wing eventually becomes capable of overcoming the vehicle weight. The performance enhancement potentially available from the wing is hence limited.
The Pegasus.TM. is carried by its launch aircraft, by direct attachment either to an underwing pylon or a special fitting beneath the aircraft fuselage. Other proposed launch vehicles which are intended to be launched by an aircraft are all designed to be carried by the aircraft in some fashion, either on top of the aircraft, under the wing, or inside the cargo compartment. Some use lifting surfaces, others do not, but in no case is there a design wherein the launch vehicle has aerodynamic lift equal to or greater than the vehicle's launch weight at an indicated airspeed equal to that of the launch aircraft.
Each of these launch vehicles suffers from the same set of deficiencies. First, the maximum weight of the launch vehicle is limited to the weight that the carrier aircraft can safely lift to the required altitude. This places an absolute upper limit on the size and weight of the spacecraft which can be launched by such launch vehicles. The weight limit is not necessarily equal to the cargo capacity of the carrier aircraft. If the launch vehicle is mounted externally to the aircraft, the interference drag added to aircraft by the addition of such appendage will require extra power to overcome. In addition, the structural loads imposed on the aircraft are greater than just the weight of the launch vehicle. The drag force on the launch vehicle and inertial load factors add significantly to the loads applied to the carrier aircraft. A structural limit may be reached long before the actual weight-lifting capacity of the aircraft has been exceeded.
Second, there is risk associated with carrying the launch vehicle, which typically contains large amounts of explosive propellant, on or in a manned launch aircraft. Explosive hazards are reasonably small during flight from the runway to the launch point. The greatest potential for explosion is during or shortly after ignition of the launch vehicle's propulsion system. Partly for this reason, most air-launch concepts require the launch vehicle to fall freely from the carrier aircraft before their propulsion system is started. This reduces the achievable reliability somewhat, in that the launch vehicle is irrevocably separated from its carrier aircraft before it is known with certainty that its propulsion system is functioning properly. There can also be a net loss of performance compared to ground launch if the launch vehicle has no lifting surfaces, and acquires significant speed during free-fall.
Third, the separation of the launch vehicle from the aircraft can introduce dynamic loads to the launch vehicle which are in turn transmitted to the spacecraft. These loads can be very severe, and require a heavier spacecraft structure than might otherwise be needed.
Fourth, externally-carried launch vehicles are subjected to the noise from the carrier aircraft's engines, and to noise generated by the complex air flow around the launch vehicle if it projects into the freestream. This imposes random vibration on the spacecraft. Vibration levels can be higher than those imposed on a spacecraft on a vehicle launched from a ground-based pad, and last hundreds of times longer. Again, a heavier spacecraft structure may be required, and delicate instruments may have to be completely redesigned to survive.
Fifth, the cost and complexity of modifications to the carrier aircraft permitting it to carry the launch vehicle increase dramatically with launch vehicle size. In fact, such modifications may become more complex and expensive than building a launch pad, reducing the incentive to utilize aircraft launch.
Finally, there is a risk to the aircraft crew from a multitude of failures which can occur when separating a launch vehicle from the aircraft. As one example, the launch vehicle control system may fail resulting in collision with the carrier aircraft and loss of both.
While launching of space launch vehicles from aircraft has significant advantages over ground-launch, the limitations associated with current designs are significant. Most important is the limitation on spacecraft size and weight imposed by current technology. In order to more fully realize the advantages of aircraft launch of space launch vehicles, as well as reduce its cost, risks, and other limitations, a new approach is desired.